In U.S. Pat. No. 5,355,845 of J. J. Burgess et al., which is hereby incorporated by reference, there is described the realization that passbands in the radiation/absorption spectra exist in the infrared region for the combustion gasses typically present in gas-fired power generating boilers. The passbands enable infrared emissions from heated surfaces of the boiler tubes to travel through the gasses for substantial distances. The energy in these passbands may be detected and measured, and used to estimate the surface temperature of the steam tubes. The temperature history in turn affords an otherwise unavailable measure of the remaining useful life of the tube and, hence, the timing of boiler tube replacement activity.
The detecting of the infrared energy is made generally possible by employing a hollow-core sapphire waveguide or the functional equivalent which functions at the high temperatures of the flue gasses, and advantageously provides a suitably narrow acceptance angle, thus enabling emitted infrared energy to be detected from a discrete region. The discrete region may be as small as a 19-inch diameter circle when the fiber tip is placed 30 or so feet from the tube surface to be monitored.
The utility of the sapphire device or its equivalents is not readily achieved, however, unless the devices can be deployed in optimal ways in the boiler so as to minimize the number of devices and their intrusion into the boiler.